In recent years, there has been studied employment of natural energy as alternate energy for fossil fuel such as petroleum, natural gas or the like. Among them, solar energy is noted which is most stable as alternate energy for fossil fuel and in a large energy amount.
However, although solar energy is very important alternative energy, it supposedly becomes a problem that (1) the energy density of solar energy is low and (2) storage and transfer of solar energy are difficult.
In recent years, research and development of solar cells have been actively made and solar light use efficiency has been increased, but there has not been achieved sufficient recovery efficiency.
As an alternate system for conversion of solar light to energy is noted solar power generation in which solar light is reflected and condensed through a mirror and electric power is generated through obtained heat. The use of this system makes it feasible to generate electric power at any time during the day or night and the power generation efficiency is thought to be higher than solar cells from the long-term view point, rendering it feasible to perform efficient employment of solar light.
A glass mirror employing a glass substrate is currently employed as a mirror for use in solar power generation but has problems such as being rather heavier, requiring a large volume, high transport cost and being fragile. Accordingly, the use of a resin mirror as an alternate to glass makes it possible to be lighter and not generating problems such as being broken, so that a film mirror in which the mirror is produced in a film form has been noted.
A high reflectance of a mirror is required to achieve enhanced power generation efficiency. Therefore, it is preferred to use, as the reflection surface of a mirror, silver which exhibits the highest reflectance of metals but silver has the defect of being easily corroded by environmental factors. Due to this, it is designed that a thick resin layer is provided between the solar light incident side and the mirror surface to protect silver.
On the other hand, a solar power generation plant is built in an area with a high amount of daily solar radiation, for example, in the desert, where there is the problem such that a sand dust adheres to the mirror surface, resulting in a lowering of reflectance with age. Further, the temperature difference between day and night is large, so that, in an area, a large amount of dew condensation water is generated on the mirror surface in the morning time, mixed with sand or clay, and dried, which stains the mirror surface, resulting in a lowering of reflectance. Specifically, in the case of a film mirror, a resin layer is provided on the outermost surface thereof so that the surface is strongly electric-charged, resulting in increased adsorption of stains. Therefore, in a plant under construction, after water is applied to the mirror surface at regular intervals, stains are removed by brushing, however, a large amount of water is used for washing and human labor is also required for washing, resulting in increased washing costs. Further, in cases when the surface hardness of the film surface is not sufficient, the film surface is damaged after being brushed, resulting in a lowering of reflectance. Therefore, it is expected that if the surface of a film mirror is subjected to a treatment combining hardness and anti-staining, stains are less likely to adhere and flawing during washing decreases, rendering it feasible to maintain a high reflectance over long duration.
Although there were reported film mirrors for solar power generation (for example, patent documents 1 and 2), it is supposed that since an acryl resin layer is provided on the outermost surface of the film, sand or clay strongly adheres to the surface, rendering it difficult to maintain enhanced reflectance over long duration.